The present invention relates to processes of accelerating bone growth (osteogenesis) and bone tissue healing around endosseous implants. In particular, the present invention relates to self-powered devices incorporated in, or attached to a surgically inserted implant, for example a dental implant or a hip or knee implant, or devices having an external power source, the devices used for accelerating bone growth and healing in and around the implant surgical site. By “self-powered” we mean devices that include a built-in power source such as a battery. The following description deals in detail with both dental and orthopedic (non-dental) implants, e.g. hip implants, knee implants, etc.
A major concern for all implants, and in particular non-dental implants such as hip or knee implants, is that external appearance, feel, and mechanical integrity and function remain essentially unchanged. Moreover, a surgeon implanting for example a hip implant will prefer to stick to existing procedures even if the implant itself were altered. Presently used implants have undergone decades of development to be brought to an optimal design. The stringent requirements of implants in terms of long term function mean that this optimal design must be preserved as much as possible in any effort to “functionalize” the implant for osteogenesis and osseointegration promotion.
It is known that dental implants are widely used, and manufactured by a number of companies (e.g. Nobel Biocare USA, Inc., 22715 Savi Ranch Parkway, Yorba Linda, Calif. 92887). Dental implants replace the natural tooth roots as anchors for the restorative device. As such, they must be well integrated into the hard bone tissue. The conventional procedure for inserting a dental implant includes drilling a hole in the maxillary or mandibular jawbone, and inserting the implant in the prepared hole. Various types of endosseous dental implants are used, e.g. blades, screws, and cylinders. The implant is generally made of titanium or titanium alloy and the top of the implant is provided with mating means (usually a top portion and inner threads) for attaching the restorative device. Before attaching the restorative device, however, there is typically a healing phase of between three to six months, during which time bone tissue grows around the implant so that it becomes well integrated with the adjacent bone. This is when direct bone-to-implant interface has been achieved. However, the implant is still at a risk of failure and crestal bone loss within the first year, some of the main reasons being poor bone strength at the interface, and low bone-to-implant contact ratio. The primary goal of osteogenesis and osseointegration as related to implants is to increase bone density and implant-bone contact ratio around any new implant as a routine common clinical practice.
During the initial and primary healing phase, a cover screw is usually attached to the top of the implant to maintain the integrity of the top portion and inner threads of the implant. After the healing phase is completed and bone integration has successfully occurred, the cover screw is removed and discarded and the restorative phase of the treatment can be initiated. In the initial bone-healing phase, woven bone is formed around the implant. This type of bone is only partly mineralized, and therefore less able to withstand the high magnitude forces applied on the implant. The 3-6 month delay between the time of insertion of the implant and the time when a restoration can be made is needed in order for the woven bone to mature and mineralize. The delay is needed because it usually takes this length of time for the bone-forming cells and bone tissue surrounding the implant to mature sufficiently to adequately hold the implant, so that the final restoration will be firmly and properly anchored. This delay is a clear disadvantage of the conventional procedure in use today, leaving the patients with impaired oral function and esthetics because of the missing teeth. The goal of the restorative dentist is to restore normal function and esthetics with no delay, therefore a dual-function device is needed: 1) for osteogenesis and osseointegration promotion to fasten and ensure implantation success and 2) a prosthetic design that allows for immediate tooth restoration. Such a dual-function device is not known in the art.
It is also known that orthopedic prosthetic un-cemented components are widely used alternatives to conventional cemented prostheses. For example, a hip joint replacement offers successful rehabilitation of damaged joints. The prosthesis can be cemented or un-cemented. The cemented prosthesis is held in place in the femoral bone by acrylic polymer cement. Crack fatigue in the cement layer and osteolysis can lead to prosthesis loosening and eventual failure. In the 1980s, a new implant design was introduced, to attach directly to bone. It was hoped that cementless prostheses would solve the problems of the cemented prostheses. For un-cemented prostheses, a very exact preparation is needed because bone cannot bridge a gap of more than 2 mm.
Longer time periods are needed for the rehabilitation process because bone must be allowed to grow towards and into the prosthesis. The un-cemented prostheses are implanted in all the patient population, but are recommended mainly for younger and more active patients. The un-cemented prosthesis may become loosened if a strong bond between stem and bone is not achieved. A long-term successful bond makes the uncemented prosthesis superior to the cemented acrylic polymer-dependent prosthesis.
The un-cemented orthopedic implant also needs bone in-growth into the porous surface of the weight-bearing part of the prosthesis (W. H. Harris, “Bony ingrowth fixation of the acetabular component in canine hip joint arthroplasty”, Clin. Orthop, 176; 7, 1983). Animal studies have shown that only 10% of the prosthesis surface is occupied by bone after three months. Bone ingrowth into human prostheses may be even smaller, one of the reasons being the large loads applied by the patients. Cook et al, in “Histologic analysis of retrieved human porous coated total joint components”, Clin. Orthop. 234; 90 1988, have found almost no bone ingrowth into the porous surface of prostheses retrieved from human patients.
It has long been known that the application of electric currents (electric stimulation) can speed bone growth and healing. The electrical stimulation may employ faradic, inductive or capacitive signals. In the mid-1960s, C. A. L. Bassett and others measured the weak electrical signals generated by the bone itself, analyzed and reproduced those signals artificially, and used them to reverse osteoporosis or aid in the healing of fractured bones. E. Fukuda in “On the piezoelectric effect of bone”, J Physiol. Soc. Jpn. 12:1158-62, 1957, and Yasuda, J. Kyoto Med. Assoc. 4: 395-406, 1953 showed that stress induced on crystalline components of bone produced current flow. Yasuda showed that similar electric signals could enhance fracture healing. Direct current capacitively coupled electric fields and alternately pulsed electro magnetic fields affect bone cell activity in living bone tissue. Friedenberg et al. in “Healing of nonunion by means of direct current”, J. Trauma, 11:883-5, 1971, were the first to report healing of nonunion with exogenous current. Brighton et al, in “Treatment of recalcitrant nonunion with a capacitatively coupled electric field”, J. Bone Joint Surg. Am. 65:577-85, 1985, reported 84% healing of nonunion with D.C. treatment. Time-varying current delivering electrodes have also been used in order to minimize accumulation of electrode products, while square wave patterns were shown to hasten mineralization during bone lengthening in the rabbit tibia. In his study, Brighton used capacitatively coupled electric fields to the limb by capacitor plates over the slim, and accelerated bone fracture healing.
K. S. McLeod and C. T. Rubin in “The effect of low frequency electrical fields on osteogenesis”, J. Bone Joint Surg. 74a:920-929, 1992, used sinusoidal varying fields to stimulate bone remodeling. They found that extremely low frequency sinusoidal electric fields (smaller than 150 Hz) were effective in preventing bone loss and inducing bone formation. They also found strong frequency selectivity in the range of 15-30 Hz. At 15 Hz, induced electric fields of no more then 1 mV/m affected remodeling activity. Fitzsimmons et al. in “Frequency dependence of increased cell proliferation”, J Cell Physiol. 139(3):586-91, 1985, also found a frequency specific increase in osteogenic cell proliferation at 14-16 Hz. Wiesmaun et al. in “Electric stimulation influences mineral formation of osteoblast like cells in vitro”, Biochim. Biophys. Acta 1538(1):28-37, 2001 applied an asymmetric saw tooth wave form at 16 Hz and found enhanced bio-mineralization. W. H. Chang in “Enhancement of fracture healing by specific pulsed capacitatively coupled electric field stimulation”, Front. Med. Biol. Eng., 3(1):57-64, 1991, showed similar beneficial results at 15 Hz to those achieved by Brighton with a 60 KHz sine-wave. Other recent references on faradic stimulation include the paper by C. E. Campbell, D. V. Higginbotham and T. K Baranowski published in Med. Eng. Phys., vol. 17, No. 5, pp. 337-346, 1995 (hereinafter CAM 95), and U.S. Pat. No. 5,458,627 to Baranowski and Black. Studies related specifically to dental bone tissue are also known, and a number of patents disclose related systems, for example U.S. Pat. No. 4,244,373 to Nachman. However, the art that relates specifically to dental bone growth stimulation by small, self powered electrical means is very limited.
U.S. Pat. No. 5,292,252 to Nickerson et al. discloses a stimulator healing cap powered by an internal small battery. The cap can be reversibly attached to a dental implant, and stimulates bone growth and tissue healing by application of a direct current path or electromagnetic field in the vicinity of bone tissue surrounding the implant, after the implant is surgically inserted. While Nickerson does not provide details of the battery, it is clear from his description that his battery is volumetrically extremely small, thus having very small capacity, which may not suffice for effective DC stimulation. Moreover, DC stimulation is known to have negative side effects. For example, Kronberg in U.S. Pat. No. 6,321,119 points out that studies on electrical stimulation of bone growth have shown that application of DC stimuli alone may be problematic in stimulating bone regeneration since bone grows near the cathode (i.e. the negative electrode), but often dies away near the anode. This phenomenon may result from electrolytic effects, which can cause tissue damage or cell death through pH changes or the dissolution of toxic metals into body fluids. Other disadvantages of Nickerson's device include: being sunken into the gingiva, it has an internal volume too small to contain a large enough battery. Its shape causes great discomfort upon removal. The healing cap is connected to the implant by a thin, weak plastic rod that may break during normal chewing. Its insulation section is larger than the battery itself, limiting the size of the battery even more.
AC (alternating current) signals may work better in electrotherapy than DC (direct current) signals, and pulse bursts may be more effective than single pulses. For this reason, many bioelectronic bone growth stimulators rely solely on AC effects, removing any net DC current from the outputs by passing the signal through a blocking capacitor. Such a capacitor forces the positive and negative output currents, when summed over a full cycle of the output waveform, to be equal, canceling each other out.
Although bone growth stimulation by AC or pulsed currents is deemed beneficial, there are no known practical, self-powered, compact dental stimulator caps using such currents. A somewhat related device disclosed by Sawyer et al. in U.S. Pat. No. 4,027,392 lacks enough description to warrant detailed discussion. Sawyer's disclosure mentions an embodiment of a bionic tooth powered by a battery and including an AC circuit that is clearly impractical: among major disadvantages, it does not appear to be removable without major surgery (since removal of his upper portion 26 occurs by unscrewing insulating member 30 from external implant thread 22, thus causing major trauma to the extensive gingival area contacted by portion 26); it uses a preferred signal frequency range of 0.5 to 1 mHz; and it cannot provide current pulses. The microcircuitry indicated by its FIG. 3 is not shown incorporated within the cap, and it is extremely doubtful that it can be implemented in the system shown. Its battery cap (“crown”) is too long, penetrating deep into the gingiva (or even through the bone), thus being unfeasible and useless from a surgeon's point of view. Also, Sawyer's device is not a dual-function device, i.e. it does not serve as a temporary abutment on which one can install a temporary crown.
Another related device is disclosed by Dugot in U.S. Pat. No. 5,738,521. Dugot describes a method for accelerating osseointegration of metal bone implants using AC electrical stimulation, with a preferably symmetrical 20 μA rms, 60 KHz alternating current signal powered by a small 1.5 V battery. However, Dugot's system is not a compact, self-powered stimulator cap, but a cumbersome, externally (to the implant) wired and powered stimulator, which does not appear to be feasibly applicable to human dental implants.
Osteogenesis devices for non-dental implants include interbody fusion devices as described in U.S. Pat. No. 6,605,089B1 to Michelson. Michelson describes a self contained implant having a surgically implantable, renewable power supply and related control circuitry for delivering electrical current directly to an implant which is surgically implanted within the intervertebral space between two adjacent vertebrae. Electrical current is delivered directly to the implant and thus directly to the area in which the promotion of bone growth is desired. However, Michelson's apparatus is not an adaptation of a readily available implant, nor does it have an optimal configuration of electrodes.
Other devices are disclosed in U.S. Pat. No. 4,026,304 to Levy, U.S. Pat. No. 4,105,017 to Ryaby, U.S. Pat. Nos. 4,430,999, 4,467,808 and 4,549,547 to Brighton, U.S. Pat. No. 4,509,520 to Dugot, U.S. Pat. No. 4,549,547 to Kelly and U.S. Pat. No. 5,030,236 to Dean, and in a recent US patent application No 20030040806 by MacDonald.
U.S. Pat. No. 6,034,295 discloses an implantable device with a biocompatible body having at least one interior cavity that communicates through at least one opening with the surroundings of the body so that tissue surrounding the implantable device can grow through the opening; two or more electrodes within the device having terminals for supplying a low-frequency electrical alternating voltage and at least one of which is located inside the cavity. U.S. Pat. No. 5,030,236 also discloses the use of electrical energy that relies upon radio frequency energy coupled inductively into an implanted coil to provide therapeutic energy. U.S. Pat. Nos. 5,383,935, 6,121,172, 6,143,035, 6,120,502, 6,034,295, and 5,030,236 all relate to the use of various materials and forms of energy to enhance the regrowth of bone at the interface between an implanted prosthesis and the native bone. None of these devices perform satisfactory osteogenesis promotion, maintenance or acceleration while leaving the implant member or stem essentially unchanged in appearance and mechanical properties.
There is thus a widely recognized need for, and it would be highly advantageous to have, practical, self-powered osteogenesis and osseointegration promotion and maintenance devices for endosseous implants that can perform electrical stimulation using various signals. It would also be extremely advantageous that such devices, when used for example in hip or knee implants, should require minimal changes to both appearance and mechanical integrity and function of the implants. The primary goal of such devices would be to increase bone density and implant bone contact ratio around any new implant as a routine common clinical practice. In the case of dental implants, such a device should preferably serve also as an abutment for a prosthetic crown that immediately restores oral function.